Worm Breeder's Gazette 13(3): 101 (June 1, 1994)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

lin-1 Vul and Let alleles: gain or reduction-of-function mutations?

Greg Beitel, Bob Horvitz

HHMI, Dept. Biology, MIT, Cambridge, MA 02139, USA

lin-1 negatively regulates vulval development and encodes a putative transcription factor containing an ETS DNA-binding domain (Beitel et al., Worm Meeting Abstracts, p. 48, 1993). We have genetically and molecularly analyzed several classes of lin-1 mutations.

Most lin-1 mutations cause a strong multivulva (Muv) phenotype characterized by a squarish protruding vulva and one to three large pseudo-vulval protrusions. About half of the animals rupture at the vulva or pseudo-vulvae during or shortly after the L4 molt. Most of the animals that survive the L4 molt are egg-laying defective (Egl) and form bags of worms if they do not rupture first. The reason for the Egl phenotype has not been determined. These mutations are recessive and do not cause any larval lethality. Southern blot analyses revealed that eight of more than 30 alleles of this class have gross alterations in the lin-1 locus. In particular, sy254 (provided by G. Jongeward and P. Sternberg) and n304 are deletions that leave only the first 40 amino acids intact and remove the exons containing the ETS domain and an additional undefined amount of coding sequence. Although there are no known deficiencies that completely delete lin-1 ,we believe these mutations to be null. We have determined partial sequences of DNA from several of the strong Muv lin-1 mutants that do not show Southern blot polymorphisms. Four of these mutants contain single-base changes in absolutely conserved residues of the ETS domain, indicating that these residues and the ETS domain are important for lin-1 function.

Three unusual lin-1 alleles that confer protruding vulvae (pVul), Egl, lethal (Let), Muv and possibly vulvaless (Vul) phenotypes were isolated by Scott Clark. The Egl and Let defects, which are similar to defects caused by Vul mutations in other genes in the vulval pathway, suggest that these mutations cause Pn.p cells to adopt non-vulval fates. This possibility is supported by the ability of these mutations to suppress the Muv phenotype caused by let-60 ( n1046 )dominant Muv mutations. Detailed lineage analyses of these mutants are in progress. These lin-1 "Let/Vul" mutations are recessive. In trans to lin-1 null allele, the penetrance of the Let phenotype is reduced while the penetrance of the Muv phenotype is increased compared with that of homozygotes. Thus, these mutations might cause increase-of-function-like phenotypes when homozygous but cause loss-of-function-like phenotypes in trans to a null allele. These observations indicate that these mutations are not hypermorphic. We propose that they are either recessive gain-of-function mutations or partial reduction-of-function mutations. Interestingly, partial reduction-of-function alleles of other genes in the vulval pathway may also have effects that are opposite those of complete loss-of-function: putative partial loss-of-function mutations in let-23 , lin-2 , lin-7 ,and lin-10 cause a Muv-like hyperinduced (Hin) phenotype, whereas strong loss-of-function or null mutations cause a Vul phenotype. An intriguing model to account for the Let/Vul phenotype of the unusual lin-1 mutations is that these mutations cause a phenotype opposite to the Hin phenotype: too few instead of too many Pn.p cells adopting the vulval fate in response to partial loss of lin-1 activity. We have determined partial sequences of two of these unusual lin-1 mutants. One contains a stop codon at position 352, and the other has an alteration of the last exon splice junction (~ position 379). Both of these mutations probably lead to truncated LIN-1 products, indicating that the lin-1 C-terminus plays an important role in lin-1 function.